Conventionally, an electric car and a hybrid car wherein the vehicle is driven using electric power stored in a battery such as a lithium-ion battery or a nickel-hydrogen battery have been developed. Especially, the lithium-ion battery that exhibits a high energy density is easy to promote the reduction in size and weight in comparison with the battery capacity. It is expected that demand for the lithium-ion battery increases as a battery for driving a vehicle for which high electric power is desired.
A popular lithium-ion battery mounted on a vehicle includes an airtight and watertight structure by which electrodes and electrolyte are sealed in a case made of metal. For example, Patent Document 1 discloses a battery pack wherein a plurality of unit cells (battery cells) is hermetically closed in a metal case sealed from the external air. It is described that the rigidity of the unit cells can be improved by using a metal case in this manner.
Incidentally, as the total capacity of a battery mounted on a vehicle increases, the energy that can be used increases, and therefore, the cruising range of the vehicle (maximum distance over which it is estimated that the vehicle can travel by the amount of charge by a single time charging operation) increases. On the other hand, since the vehicle weight increases as the mounted quantity of batteries increases, the energy spent for driving the vehicle increases and both the rate of electric consumption and the cruising range decrease. Accordingly, if the weight of the battery itself can be reduced, then it becomes possible to utilize a greater amount of energy as the driving energy for the vehicle and improve the traveling performance of the vehicle while curbing the vehicle weight increase.
Therefore, in recent years, it has been proposed to use, as a case for a battery, a case made of a resin (plastic, resinoid) in place of a metal case. For example, as described in Patent Document 2, it has been studied to form a cell battery case (vessel) for a battery as a polypropylene case to assure requested rigidity and achieve reduction in weight and cost.